APC vs. AFC: confused

Dear all,

Can you please help me. We have troubles with our optical fiber. First of all the beam emerging from the fiber shows a strange pattern when viewed on a screen and second the transmission changes upon changing the way the fiber runs on our table (we do not affect our fiber coupler when doing this changes). Moreover we lose very much power in the fiber. Therefor we think the fiber is out of order and should be replaced as soon as possible. Is it possible that we damaged our fiber by using a wavelength 10nm below the cutoff wavelength(457nm used at 470nm cutoff). Or did we just break it or scratch it? This fiber has on one end an AFC connector. But fibers with this feature are rare. So my question is if we can replace this fiber with a fiber that has an APC on one end even if the fiber coupler (oz optics) is designed for the use with an AFC?

Thank you in advance regards reinhard

Reply to
JoS
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It sounds like there are multiple things going on here. First, there might be dirt on the end of your fiber. If you are doing fiber work and having trouble you need to invest in a fiber inspection microscope so you can see if there is dirt on the end of your fiber or if you have blown out the connector.

You will always change the transmission when you rearrange the fiber. It is particularly noticeable if you 1) have a polarization sensitive setup 2) are beyond the bend radius of the fiber 3) aren't at the optimum wavelength for transmission through the fiber. I recommend that you use fiber for the wavelength designed; find out what the bend radius of the fiber is for that wavelength and don't curl it tighter than that; and most importantly tape everything down and leave it there. Heat, vibration and sound will all couple into the fiber so watch for air drafts and vibration sources.

Don't mix connector types. AFC fiber is for free space to fiber coupling. APC is for fiber to fiber coupling. If you mix the two your coupling efficiency will be worse. If you have any questions about the device you are coupling to call OZ directly. There are many white papers on their web site that teach about fiber connector types and coupling into fiber. It sounds like you need to do some reading.

Good luck, P. Danek

JoS wrote:

Reply to
danek

Thank you for your fast reply. I managed to examine the fiber on a microscope (fiber fixed above objective, illumination via the fluorescence excitation light path, using a YFP filter cube) and I think there are structures that look like dirt on the surface, even on the dark spot, which is as i think the core, I found some dirt. when i illuminate the other end of the fiber I see a small bright spot within the "core" on this place I did not find dirt-like structures. What ways are there to clean the fiber? lens cleaning paper and isopropanol?

regards reinhard

danek wrote:

Reply to
JoS

Is the fiber a single mode fiber?

You wouldn't damage the fiber with normal power levels that are beyond the cutoff but if you're using a single mode fiber, you might "see" more than one mode by operating it past the single-mode range.

How are you launching into the fiber such that you know you're losing significant power? Since you're using visible laser, any significant loss would cause an unjacketed fiber to visibly glow at the point or area of loss.

Reply to
John_H

Thank you for your reply. Its a polarization maintaining and jacketed fiber. Does this mean that the pattern we see in the output beam can be different modes? may be i was not clear enough concerning two different things. one is the pattern, this can be seen with any wavelength we used. the other is that we found out that the fiber we are using has a cutoff higher than our shortest wavelength. but i am glad we did not damage our fiber using shorter wavelength, as it might be in the case we have to replace it we have to use a fiber with similar characteristics. a further question arising thereof: what parameter is described by the cutoff wavelength? We know about presumed losses from laser power measurements. we are coupling in with a power of ~30mW and get max 10uW at the other side of the fiber. we tried several times to align the fiber according to the protocol but these 10uW were the maximum. this is far below any expected value.

John_H schrieb:

Reply to
Reinhard Fritsch

If it is PM fiber then you will need a polarization rotator between the light source and the fiber. Your coupling loss is more than likely due to a misalignment of the fast vs slow axis. The easiest way to do this is to launch into SM fiber that has a polarization rotator attached to it. You attach the PM fiber to the output of the rotator/sm fiber. Tape everything down. Use a power meter at the end of the PM fiber to optimize the rotator position. Now you are launching into the correct axis of the PM fiber and you can continue to add polarization maintaining devices. Twisting fiber changes the polarization characteristics of it so everything must be fixed to an optical breadboard or bench.

The dark ring that you see in the microscope is the cladding. When you illuminate the fiber from the other end (hopefully not with a laser; you will cause eye damage that way) and see a small bright spot that is the core.

30mW is low power and you shouldn't have damaged anything unless you curled it too tightly.

Cutoff wavelength is the lambda that is readily supported by the core. You can go outside of this parameter but you will possibly couple into the cladding. In which case you will get higher losses at bend sites since light coupled into the cladding is easily lost at fiber bend sites. Another thing that can happen is that you launch multiple modes into the fiber. And you will also shorten the distance that you can coherently travel down the fiber. It is best to match wavelength to the fiber to avoid all of that nastiness. If you are using short lengths of fiber you might be able to get away with it but it sounds like you are having problems.

Clean the fiber ends with non particulating lens tissue or clean room wipes and optical grade isopropynol. Do not let the alcohol touch your fingers since you will wick the oil onto the surface you are cleaning. Dirt does not need to be blocking the core to cause a problem since a bit of dirt can cause the connectors to stand off from each other and not come into contact properly for good coupling.

P. Danek

Re> Thank you for your reply. Its a polarization maintaining and jacketed > fiber.

Reply to
danek

Thank you again for your opinion. I tried to rule out all possible flaws. I think that the polarization is all right in out setup as we couple into the fiber after an AOTF which allows us to select the wavelength and produces a beam with E || to the base. This means E || to the slow axes. In this configuration we have been able to get a far better coupling efficiency in the past. I even tried turning the coupler by 180° to see if the fiber was twisted. Using the FC side and not the AFC side of the fiber did not bring better results. Thus ruling out a damage of the fiber face. Of course I was aware of the far worse efficiency a FC end has compard to an AFC end. We are working on a breadboard. I can reassure you I did not use the Laser for illuminating the fiber on the microscope. I used the transmitted light illumination form the microscope instead. Using the CFP cube I have been able to see the PANDA structure of the fiber. I hope that by now I have tested all possible causes for our poor coupling efficiency. But by now I did not fond the cause of it.

regards, reinhard

danek wrote:

Reply to
JoS

Lets cover this again

You are operating a single mode fibre below its cut off wavelength. This means that the fibre is no longer considered single mode, the fibre is going to be cabable of supporting at leasty two modes. How much power is in each mode will depend on many factors, and small perturbations will cause cause coupling between the modes. To complicate matters the same perturbations can cause coupling between the the two polarisation states as well.

That pretty much takes care of the strange pattern on your screen and the fact that it changes when the fibre is moved. There is no great surprise there.

Your coupling efficieny on the other hand is execptionally poor. To get an idea of how this can be improved it is useful to know

  1. what is the source you are using and what are its output beam properties.
  2. what lens arrangement are you currently using..
  3. what is the numerical aperture of the fibre. ( in fact what is the fibre, how long is it and what is the attentuation coefficient)

I would expect that the coupling efficeincy can be improved by a coupleof orders of magnitude, however this may be academic, if you are looking for a stable output beam, given your current operating wavelength.

cheers

David

Reply to
David M

Thank you for your reply. Another thing I did not define well in this discussion was that we observe this pattern even when we used 514nm which is well above the cutoff of 470nm. ad 1)As a source we use an Argon multi wavelength Laser. The beam is modified by an AOTF which selects the wanted lambda and turns the polarization by 90°. We had an old setup where the laser the AOTF and the coupler were in line. Now we changed to a setup where we employ two mirrors for the adjustment of the beam. So our source now is this laser then two mirrors then the AOTF and finally the coupler. So the beam should be polarized parallel to the breadboard and parallel to the slow axes. We changed the setup because the old one was not stable. And now with the new one we do not get much light through the fiber. ad 2)Besides the lenses in the coupler (focal length: 3.5mm, achromat lens) there are no lenses in the optical path prior to the coupler. ad 3) I have not been able to find out the numerical aperture and the attenuation coefficient of our fiber. It is a polarization maintaining fiber (PANDA). The length is 3m. With the old setup we go much higher output powers after the fiber.

regards reinhard

David M wrote:

Reply to
JoS

Going by a very old data sheet (~1998) and assuming you have SM48 which is listed as 0.4-0.47um for cutoff, then the following specs are listed:

center wavelength 0.48um core 4um cladding 125um atten 50 dB/km beat length 3mm extinction ratio -30 db/100m

According to the data sheet I have this was a special order fiber. There is also a high power version of at the same wavelength but all of the specs are identical. If the fiber is newer these specs may not apply.

P. Danek

JoS wrote:

Reply to
danek

The other options

it is possible that you are coupling nearly all you light into the cladding, and that although the cladding modes are rapidly attenuated, there is still a residual at the output of your relativeyl short fibre. The pattern would be more speckle like with this theory, whereas I suspect that a coupling of different polarisation modes would have reasonabley defined lobes.

Of course as you first pointed out your fibre could be damaged.

To check this I'd strip the system down to the laser and fibre coupling arrangement, and test with no AOTF or mirrors so that these can all be eliminated as potential causses of your problem. As you have achieved resonable coupling before it is good to that stage to determine wher thisngs could be going wrong, and then add one component at a time.

cheers

David

Reply to
David M

You mentioned that the fiber is jacketed. Depending upon the type of jacket you might be able to carefully remove it then you will be able to inspect it. This is done by passing the fiber between your fingertips which are very sensitive. If you feel any bumps look more closely at the location. The fiber should be smooth end to end. I have had problems with the connectors not being installed properly but they were put on by a company other than Fujikura. To check this tug gently on the connector. My bad ones would come right off even with a gentle pull. You could also try wiggling the fiber at the connector to see if it is damaged. If there is a problem you should see the power levels fluctuate drastically while gently moving the fiber.

P. Danek

David M wrote:

Reply to
danek

Thanks to all of you for your kind help. We now made some progress. After stripping down our system to just the laser and the coupler we were able to get good transmission (about 18mW of 30mW)(at least our fiber is in a good shape). After reinserting of the AOTF we were not able to get such a good result. When we tried to reproduce the good coupling we achieved without the AOTF we failed. Since this one glimpse at what is possible we are in the dark again. So I suspect that it is something with the coupler. But we can not think what that might be. We tried different angles of incident onto the coupler (+/- ~2°-3°)or various insertion depth of the fiber into the coupler (searching for the focus of the coupler lens) but we were not able to find the right position. Without AOTF we can see the different lambdas comprised in the beam in separate patterns. We are on the borders of our knowledge, but we have to find a solution. I will tell you if got got it working in the end. Thanks again.

Reinhard

danek wrote:

Reply to
JoS

In this case I would suspect that the AOTF is the problem.

Either you're losing significant power in there or it is messing with your beam, perhaps increasing the divergence, or a combination of both, or it simply isn't correctly aligned. All of thatshould be pretty easy to check by

1=2E measuring the actual power after the AOTF 2=2E checking the beam shape by projecting onto a pieace of card and varying the distance

cheers

David

Reply to
David M

I'd be looking closely at the beam deflection and walkoff in the AOTF. Can you shoot the beam across the room and see if it deflects when you install the AOTF?

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Again thanks for your help. We finally have been able to get a Gaussian profile out of the fiber. So there is light at the end of the tunnel. The adjustment was far more sensitive than we thought. For today we were about 10 fold below our expectation. But I am positive to get better results tomorrow. Regards Reinhard

Reply to
JoS

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